Conventional methods for compressing and encoding data, and digital data in particular, continually strive for higher throughput rates. In transmission schemes, for example, digital data is typically converted into a binary baseband signal wherein the data to be transmitted can be viewed as a stream of 0's and 1's, as shown in FIG. 1.
Computer modems, for example, employ modulation techniques such as Phase Shift Keying and Quadrature Amplitude Modulation, whereby a pattern of bits (i.e., a binary baseband signal such as that shown in FIG. 1) is used to modulate a carrier signal. Where a tribit PSK coding is employed (for example, that recommended by CCITT Rec. V.27), three bits at a time are used to phase shift the transmitted signal by up to eight phases, permitting up to three times the bandwidth of the telephone carrier to be transmitted, and achieving 9600 bits/sec throughput. FIG. 2 illustrates PSK modulation in a conventional computer modem; however, for convenience of illustration, only a dual phase technique (unibit coding) is shown.
As the ability of computers to process information continues to advance, however, so does the need for higher transmission rates of data. Full-motion video, hypertext, and sound wave synthesis capabilities are becoming commonplace in even standard-grade computer systems, and these applications require huge quantities of data at high throughput rates. Consequently, transmission of these amounts of data at useful speeds is becoming a serious bottleneck to the development and proliferation of this technology.
Transmission of data across telephone lines is limited most notably by the narrow bandwidth available with telephone equipment. The useful range is generally limited to between 300 Hz and 3000 Hz, yielding a bandwidth of less than 3 kHz. Conventional 9600 baud modems using PSK techniques on binary baseband signals swallow virtually all of this usable bandwidth, constraining the ability to improve data throughput.
It should be noted that other means for transmitting digital data also generally rely on a binary baseband signal and thus suffer from the same bandwidth constraints, albeit to different degrees, as telephone transmissions. The transmission of data over a telephone system is given merely as one example where the conventional technology suffers from the problems described above. Other areas where similar constraints apply include local and wide area networks, satellite communications, mass storage devices, radio and cellular communications, digital audio and video recordings, and so on.